Proposal: Early ventilator weaning to HFNC in hypoxemic respiratory failure

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A previously healthy 45-year-old man was transferred to the Genius General Hospital ICU for management of pneumonia.He was intubated prior to transfer due to hypoxemia (details unavailable).His chest radiograph showed dense right lower lobe consolidation, which was confirmed with ultrasonography.He was treated with a regimen of dexamethasone, ceftriaxone, and azithromycin as discussed last week.

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On the second hospital day his oxygenation requirements were stable, requiring 55% FiO2 on 5 cm PEEP.His chest radiograph showed stable consolidation of the right lower lobe without any progression of the pneumonia.He was comfortable, calm, and awake on the ventilator.

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Based on the ICU weaning protocol, this patient did not qualify to undergo a spontaneous breathing trial because he was requiring >50% FiO2.Nonetheless, a spontaneous breathing trial was performed on 55% FiO2.He tolerated this well and was extubated to high-flow nasal cannula (HFNC) set to 60% FiO2 and 50 liters/minute flow.Over the next few days HFNC was gradually weaned off.

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Introduction

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There is little data regarding how well a patient must oxygenate before considering extubation.The most recent guidelines recommend delaying extubation until the patient can tolerate ?40% FiO2 (Boles 2007).Nonetheless, many modern weaning protocols and literature allow for extubation on ?50% FiO2 (e.g. UPenn protocol).With the availability of high-flow nasal cannula it might be possible to consider extubation in very carefully selected patients requiring >50% FiO2..

Intubation-extubation paradox: Failure of rigid extubation criteria

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Imagine what would happen if guidelines requiring FiO2 ?40% before extubation were followed for a patient with a baseline oxygen saturation 85-90% on six liters nasal cannula (which generates an inhaled oxygen concentration of ~44% FiO2).If this patient were intubated for any reason (e.g. seizures, elective surgery), it would be impossible to ever reach extubation criteria!Of course, in reality, if nothing changed with the patient's respiratory system, it should be possible to extubate the patient back to their chronic home oxygen prescription.This illustrates that extubation criteria should not be applied rigidly, but instead may require adaptation to the clinical scenario.

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HFNC provides administration of higher levels of FiO2 than previously practical following extubation

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Prior to HFNC, the highest fraction of inhaled oxygen (FiO2) that could be provided after extubation for extended periods of time was around 50% via a Venturi mask.A non-rebreather facemask can increase the FiO2 to 60-70%, but this may vary depending on the respiratory pattern and mask seal.Noninvasive ventilation can provide 100% FiO2, but generally cannot be tolerated for more than a couple days continuously. Using these devices, it would be difficult to extubate a patient requiring >50% FiO2.

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HFNC changes this by allowing longer-term powerful support of oxygenation (e.g. >90% FiO2 and ~5 cm PEEP).This might allow extubation of a patient requiring >50% FiO2 while still leaving a margin of error, in case the patient's oxygenation deteriorates. HFNC is more comfortable than BiPAP, with some patients remaining on it continuously for days to weeks.

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Hypoxemia alone is usually not the cause of post-extubation respiratory failure

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Provided that the underlying lung disease is stable or improving, isolated hypoxemia rarely causes re-intubation.The patient's oxygen requirements may be estimated reasonably well prior to extubation based on FiO2 and PEEP on the ventilator.The clinician can subsequently provide a similar or slightly higher FiO2 after extubation.Overall this may explain why most studies show no relationship between oxygenation and risk of re-intubation (Thille 2013). (One notable exception to this is cardiogenic pulmonary edema, which may be exacerbated by the withdrawal of positive intrathoracic pressure.)

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The most common causes of post-extubation respiratory failure are respiratory muscle fatigue and inability to clear secretions from the airway.Respiratory muscle fatigue may be predicted on the basis of the spontaneous breathing trial (e.g. rapid-to-shallow breathing index, tidal volume, respiratory rate, required minute ventilation).Ability to clear secretions from the airway is harder to assess, but may be predicted on the basis of multiple factors (cough strength, sputum volume, suctioning frequency, and mental status).Although failure from these causes is often associated with hypoxemia, the hypoxemia is a secondary problem (e.g. due to mucus plugging or atelectasis).

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Symmetric nature of intubation and extubation

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Although we may conceptualize intubation and extubation separately, they are two sides of the same coin. For example, readiness for extubation is determined largely based on a rapid-shallow breathing index during a spontaneous breathing trial (respiratory rate divided by tidal volume). Alternatively, for a patient with respiratory failure approaching intubation, one of the most helpful vital signs is a worsening respiratory rate. In both cases, a high respiratory rate usually reflects an imbalance between diaphragmatic strength and respiratory workload, signaling impending diaphragmatic failure.

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Similarly, the same techniques that may help avoid intubation can be used to facilitate early extubation.The classic example would be BiPAP in the setting of COPD.Early application of BiPAP in COPD reduces the intubation rate.Selected COPD patients who require intubation can be aggressively extubated after 48 hours directly to BiPAP support, even if they do not meet the traditional criteria for extubation (Nava 1998).Thus, BiPAP can be similarly helpful for either avoiding intubation or facilitating extubation (discussed further by the LITFL blog here).

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HFNC was shown in the FLORALI study to reduce the intubation rate in hypoxemic respiratory failure. It may be imagined that HFNC provides a greater level of respiratory support than low-flow oxygen.Thus, for some patients with moderate severity illness, traditional oxygen support would be insufficient, leading to intubation (black arrow below).However, HFNC provides sufficient support to avoid intubation entirely:

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As shown below, a patient with more severe disease may require intubation even with HFNC support.However, HFNC might still facilitate early extubation to a greater level of noninvasive respiratory support:.

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Evidence regarding the use of HFNC following extubation

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Evidence up until August 2014 was previously explored in this post. The most notable study was Maggoire 2014, which randomized 105 hypoxemic patients following extubation to HFNC vs. oxygen via Venturi mask. Patients randomized to HFNC experienced less tachypnea, lower PaCO2, and lower reintubation rates (4% vs. 21%, p = 0.01)..
Since then, Stephan 2015 explored the use of HFNC in patients extubated following cardiothoracic surgery. BiPAP has been shown to reduce reintubation in this setting, so this trial was designed involving 830 patients randomized to BiPAP vs. HFNC. The two groups had nearly identical rates of reintubation. It is unclear exactly how this extrapolates to non-surgical patients, but overall the study supports post-extubation HFNC.

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There is no evidence regarding extubation of a patient requiring >50% FiO2 to HFNC.Thus, this post is merely a proposal.

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Conclusions

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There is little evidence regarding how well a patient must be able to oxygenate before liberation from mechanical ventilation.Most studies do not find a strong relationship between oxygenation and the risk of extubation failure.

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HFNC has the capacity to provide high levels of oxygen support in a comfortable and stable fashion.This could provide a safety net, possibly allowing us to extubate patients who are a bit more hypoxemic than we might otherwise feel comfortable with.However, until evidence is available, this must be considered very cautiously in highly selected patients.A potential candidate might have purely hypoxemic respiratory failure with improving underlying disease and all other indicators strongly predictive of a successful extubation (e.g. excellent performance on spontaneous breathing trial, strong cough, normal mental status, no evidence of volume overload).